Thanks to the member that gave me a link to the man in seat 61, which I thought was only trains. Scroll down for details of the bus Phnom Penh to Sihanoukville.
http://www.seat61.com/Cambodia.htm#Phno ... lle_by_bus
There is a ton of valauble info ffor all travellers to cambodia.
further down the page was this piece of interesting news
Train service to be revived by 2013?
But Cambodian & foreign backers plan to bring back Cambodia's railways from the dead. A company called Toll Royal Railway (www.tollroyalrailway.com) has been given a 30 year concession to repair and operate the railway, and by 2013 it's planned to reopen both the Southern Line from Phnom Penh to Kampot & Sihanoukville (254 km) and the Northern Line from Phnom Penh to Battambang, Sisophon & Poiphet on the Thai border (388 km). Indeed, I have witnessed the new ballast and sleepers being laid between Sisophon and Poipet myself, in late 2011. It could mean passenger trains linking Bangkok & Phnom Penh once more, after decades! One freight service is already back up & running, and the rehabilitation of Phnom Penh's historic main station is already under way.